The present invention relates to a continuous process for the preparation of organic isocyanates by reacting primary organic amines with phosgene in an organic solvent at temperatures of from 60.degree. to 100.degree. C., wherein the reaction mixture is circulated by means of a bubble column.
The continuous preparation of organic isocyanates by reacting primary organic amines with phosgene in an organic solvent is frequently described and is carried out on a large scale (see, for example, Ullmanns Enzyklop adie der Technischen Chemie, 4th Edition, Volume 13; Kunststoffhandbuch, Volume 7 (Polyurethane), 3rd revised Edition, Carl Hanser Verlag, Munich, Vienna, page 76 (1993). The course of the reaction is, in most cases, separated into two temperature steps, the cold and the hot phosgenation. In the first step, the amine is reacted with phosgene in a rapid exothermic reaction at low temperatures to form a mixture of carbamic acid chloride and amine hydrochloride. This reaction mixture is then "completely phosgenated" at elevated temperatures, until the evolution of gas has stopped. A satisfactory yield is achieved in this process through the large excess of phosgene, through the dilution of the co-reactants using a large amount of an organic solvent, and through the two-step reaction procedure (cold and hot phosgenation).
The disadvantage of this process is the high energy consumption, which is necessary due to the recovery and purification of the solvent and the excess phosgene. For this reason, there have been many attempts to improve the phosgenation process.
Thus, in the patent literature a series of processes is described in which the purpose is to ensure a very efficient, thorough mixing of the starting components reacted together--primary amine and phosgene--with circulation apparatus and/or dynamic or static mixers being used. By this means, it is possible to operate at a higher concentration (less solvent) and using a lower phosgene excess, with the same yield being obtained. For example, German Offenlegungsschrift 2,212,181 describes a process in which intimate mixing is achieved by pumping the liquid reaction mixture together with the gaseous reaction products cocurrently in a recycle loop through a packed column at a rate such that a so-called transition flow having turbulent properties develops in the packed column. German Offenlegungsschrift 2,624,285 describes a process whereby the reaction mixture together with the liquid phosgene are pumped in a recycle loop through a jet nozzle, with a solution of the amine being fed to this nozzle in such a way that a reaction and mixing zone having a high energy dissipation density is formed. Another circulation process is described in German Offenlegungsschrift 3,212,510. In this patent, the process is operated under pressure at a temperature such that 30 to 70% of the carbamic acid chloride formed in the first reaction step is decomposed into isocyanate, so a low-viscosity mixture is produced, which can be pumped around at a high velocity. In U.S. Pat. No. 3,781,320, a circulation apparatus is described for the preparation of organic isocyanates from primary amines and phosgene, wherein a mixing unit having a higher shearing action is employed.
The process for the phosgenation of amines in a circulation apparatus is described in the German patent 1,037,444. Here, the mixing of the co-reactants takes place under pressure and at a temperature higher than the decomposition temperature of the carbamic acid chloride. Since the separation of hydrogen chloride and excess phosgene occurs at low pressure, a pump having a suitable delivery head is used. Several of the previously known processes using circulation apparatus have the disadvantage, however, that they are operated at temperatures at which the carbamic acid chloride formed in the first reaction step is to a great extent partially or completely decomposed into isocyanate and hydrogen chloride and where the free isocyanate competes so strongly with phosgene during the reaction of the amine that ureas are formed. These formed ureas are no longer completely converted to isocyanate but lead to by-products which diminish the yield of isocyanate and/or impair the quality of the product.
In other phosgenation processes, attempts are made to achieve the desired intensive, thorough mixing by means of dynamic or static mixers in a single passage, that is, without circulation. Thus, German Offenlegungsschrift 2,153,268 describes the reaction of primary amines with phosgene in organic solvents in multistage centrifugal pumps and German Offenlegungsschrift 3,121,036 describes the same reaction in a smooth jet nozzle.
Another way, at least of lowering the phosgene excess used, is phosgenation under pressure. To ensure an economic production output, all large-scale processes for the preparation of organic isocyanates by phosgenation of primary amines operate at elevated temperatures. The solubility of the phosgene in the reaction mixture and with it, the active phosgene excess are decreased in this process. An increased reactor pressure counteracts this. Apart from the fact that the high dilution necessary for a good yield is not decreased during these processes, large-scale pressure apparatuses are very expensive and present problems due to the high toxicity of phosgene.
The carrying out of the phosgenation reaction under pressure also has the disadvantage that, besides the phosgene concentration, the HCl concentration also increases in accordance with Henry's law and the formation of amine hydrochloride is thereby favored. The advantage of the higher phosgene concentration in connection with the reaction rate is completely or partially offset thereby, as the reaction of amine hydrochloride with phosgene is the rate-determining step in any phosgenation process for the preparation of isocyanates.
All the previously known processes for the reaction of primary amines with phosgene in circulation apparatus have moreover the disadvantage that pumps are required for the circulation.